I observe this funny behaviour in impress. When I have a video crossing the boundaries of the slide, during the presentation slide slowly zooms in (not zoom out, which might make the video to be displayed fully). It doesn’t matter if the video is embedded or linked.
Build ID: 1:6.1.3~rc2-0ubuntu0.16.04.1
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk2;
Locale: de-AT (en_US.UTF-8); Calc: group threaded